Probing method using ion trap mass spectrometer and probing device

ABSTRACT

An analyzing method which includes ionizing a sample, dissociating plurality of precursor ions which have different m/z values, and deciding if fragment ions of a predetermined m/z value are present.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/123,202, filed May 6, 2005, which is a continuation of U.S.application Ser. No. 10/311,270, filed Dec. 13, 2002, now U.S. Pat. No.6,894,276, which is a 371 of PCT/JP00/06411, filed Sep. 20, 2000, thecontents of which are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to techniques for detecting explosives anddrugs, and particularly to a detecting device using ion trap massspectrometer.

BACKGROUND ART

Detecting devices are demanded for detecting explosives in order toprevent terrorism and maintain security in the midst of the aggravatedinternational conflicts. Baggage inspection apparatus using X-raytransmission are now widely used as detecting devices chiefly inairports. Since the X-ray detecting device detects objects as lumps andidentifies dangerous materials from information of shape, it is calledbulk detection. The detecting method based on gas analysis is calledtrace detection, and used to identify substances from chemical analysisinformation. The trace detection is characterized in that very smallamounts of ingredients attached to a baggage or the like can bedetected. A security scanner of greater precision is desired to produceby combining bulk detection and trace detection in order to sociallystrengthen the security.

Custom offices also use detecting devices in order to detect forbiddenchemicals sneaked through various routes. Although bulk detectors anddrug-sniffing dogs are chiefly used in custom offices, a trace analyzerfor forbidden chemicals is eagerly desired to produce in place of thedrug-sniffing dogs.

Various analyzing methods such as ion mobility spectroscopy and gaschromatography have been tried as trace detection. Research is beingconducted for developing apparatus having high speed, high sensitivityand high selectivity as important factors in the detecting device.

In these situations, a detecting method based on mass spectrometry isproposed that is fundamentally excellent in speed, sensitivity andselectivity as, for example, disclosed in JP-A-7-134970. A conventionaldetecting device based on mass spectrometry will be described withreference to FIG. 16.

An air suction probe 1 is connected through an insulation pipe 2 to anion source 3. The ion source 3 is connected through an exhaust port 4and an insulation pipe 5 to an air exhaust pump 6. The ion source 3 hasa needle electrode 7, a first aperture electrode 8, an intermediatepressure portion 9 and a second aperture electrode 10. The needleelectrode 7 is connected to a power source 11, and the first apertureelectrode 8 and second aperture electrode 10 are connected to an ionaccelerating power source 12. The intermediate pressure portion 9 isconnected through an exhaust port 13 to a vacuum pump. An electrostaticlens 14 is disposed in the stage succeeding the intermediate pressureportion. A mass spectrometric portion 15 and a detector 16 are disposedin the stage succeeding the electrostatic lens 14. A detected signalfrom the detector 16 is supplied through an amplifier 17 to a dataprocessor 18. The data processor 18 detects a plurality of m/z values(mass number of ion/valence of ion) of particular chemicals, thusdeciding if the detected gas contains a particular chemical.

This data processor 18 has a mass determining section 101, a drug-Adetermining section 102, a drug-B determining section 103, a drug-Cdetermining section 104 and an alarm driving section 105. An alarmdisplay 19 that is driven by the alarm driving section 105 has displayportions 106, 107, 108 disposed.

DISCLOSURE OF THE INVENTION

The above prior art has the following drawbacks.

The above device detects chemicals by using the m/z values of ionsgenerated by the ion source. Therefore, when a chemical substance existsthat generates ions of the same m/z values as those of the detecteddrug, there is a possibility that an alarm is issued despite the factthat there is no drug.

More specifically, there is the problem that while the device isdetecting an awakening drug within baggage, it may misreport in responseto the ingredients of a beauty product stored in the baggage. Thismisreport is caused by low selectivity of the mass spectrometric portionfor analyzing ions, or when the device cannot distinguish the ions ofthe drug incidentally having the same m/z values from the ions ofcosmetics.

Tandem mass spectrometry is known as a method of increasing theselectivity in the mass spectrometer. As devices for implementing thetandem mass spectrometry, there are triple quadrupole mass spectrometerand quadrupole ion trap mass spectrometer. In the tandem massspectrometry, the following steps are performed:

(1) First-stage mass analysis, in which mass analysis is made to measurethe m/z values of ions generated by the ion source;

(2) Selection, in which ions having a particular m/z value are selectedfrom the ions having various m/z values;

(3) Dissociation, in which the selected ions (precursor ions) aredissociated by collision with neutral gas to produce resolvent ions(fragment ions); and

(4) Second-stage mass analysis, in which mass analysis is made forfragment ions.

When the precursor ions are dissociated, the strength of chemical bondat each place of the molecule determines where the molecule is cut away.Therefore, analyzing the fragment ions will make it possible to obtain amass spectrum containing an extreme abundance of molecular structureinformation of precursor ions. Accordingly, even if the m/z values ofions generated by the ion source are the same by chance, it can bedecided if the objects being detected are contained in pieces of baggageby examining the mass spectrum of fragment ions.

Thus, if the mass spectrometric portion 15 in the conventional probingdevice of FIG. 16 is replaced by a triple quadrupole mass spectrometeror quadrupole ion trap mass spectrometer to implement the tandem massspectrometry, the selectivity can be improved, and the misreport can bereduced. However, the tandem mass spectrometry takes a longer time thanthe normal mass spectrometry, and thus the detecting speed of thedetecting device cannot be improved.

For the above reasons, the detecting device has been requested to havehigh selectivity and high speed.

Accordingly, it is an object of the invention to provide a high-speed,less misreport detecting device for explosives and banned drugs by usingfast screening mode and high-selectivity detailed checking mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the algorithm of one embodiment of theinvention.

FIG. 2 is a diagram showing one example of the construction of adetecting device for embodying the invention.

FIG. 3 is a diagram showing one example of the construction of the ionsource for embodying the invention.

FIG. 4 is a diagram showing the construction of the vapor-samplingportion for embodying the invention.

FIG. 5 is a timing chart of the voltage application in the embodiment ofthe invention.

FIG. 6 is a diagram showing the procedure of MS/MS spectrometry in theembodiment of the invention.

FIG. 7 is a diagram showing the procedure of spectrometry in theembodiment of the invention.

FIG. 8 is a flowchart of the tandem mass spectrometry.

FIG. 9 is a diagram showing the algorithm of another embodiment of theinvention.

FIG. 10 is a timing chart of the high-frequency voltage applied to thering electrode and end cap electrode.

FIG. 11 is a diagram showing the frequency of the voltage applied to theend cap electrode during the trap and selection time interval.

FIG. 12 is a diagram showing the situation in which the ion trap anddissociation are simultaneously progressed in the embodiment of theinvention.

FIG. 13 is a diagram showing the frequency of the high-frequency voltageapplied to the end cap electrode when the ion trap and dissociation areprogressed at a time in the embodiment of the invention.

FIG. 14 is a diagram showing the algorithm of still another embodimentof the invention.

FIG. 15 is a diagram showing the construction of the device of theembodiment according to the invention.

FIG. 16 is a diagram showing the construction of the conventional massspectrometer used for detecting dangerous objects.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described in detail with referenceto the accompanying drawings.

FIG. 1 is a diagram showing the algorithm of the first embodiment of theinvention.

The detecting method of this embodiment has a first analysis step 201for acquiring mass spectrum, a first decision step 202 for deciding ifthere are ions of a first peculiar m/z value, a second analysis step 203for tandem mass spectrometry according to the decision results in thefirst decision step 202, a second decision step 204 for deciding ifthere are ions of a second peculiar m/z value, and an announcement step205 for issuing an alarm in accordance with the decision results in thesecond decision step 204. The measuring operation by the steps 201 and202 is called as the screening mode, and the measuring operation by thesteps 203 and 204 as the detailed checking mode.

To detect, in step 201 the ions generated from the sample gas areanalyzed, and in step 202 it is decided if ions of the m/z valuecorresponding to that derived from the detected object are detected. Forexample, when amphetamine as a kind of stimulant drug is ionized in thepositive atmospheric pressure chemical ionization mode, it producespseudo molecular ions (M+H)⁺(M is sample molecule, and H is proton) thatis the addition of proton to amphetamine molecule. Since the m/z valueof the false molecular ions is 136, decision is made of if ions of 136in m/z are detected in step 202 (first decision).

Here, the m/z value to be decided in step 202 is of course dependentupon the object to be detected. A plurality of different m/z values maybe decided for various different narcotic drugs and stimulant drugs.

When the analysis time in the first analysis step 201 is selected to be0.1 second, the results from the repetition of step 201 and integrationof the measured results may be used for the decision in step 202. Sincethe random noise can be averaged by the integration, the error in step202 can be reduced.

When ions of a predetermined first peculiar m/z value are decided toexist in step 202, the second analysis step 203 for tandem massspectrometry (hereafter, MS/MS) is performed. Step 203 includes theprocesses of precursor-ion selection, precursor-ion dissociation, andmass spectrometry of fragment ions. In order to increase the analysisprecision, longer time should be taken in step 203 than in step 201.

Step 203 can produce a mass spectrum with abundant molecular structureinformation. This mass spectrum is decided (second decision) in step 204to have a m/z value peculiar to the object being detected or not. If itis present, the alarm is activated to send an alarm signal.

In step 204, if the mass spectrum of the object being detected by tandemmass spectrometry is previously measured and stored as database,referring to the database can make high precision decision.

The detecting method using the algorithm shown in FIG. 1 will bedescribed below in more detail. During the detecting operation, thescreening mode (namely, measurement in step 201 and decision in step202) is repeated. If the decision in step 202 is made after theaccumulation of 10 measurements each of which is selected to take 0.1second for step 201, the total time necessary for the detecting becomesabout 1 second. If the object being detected is doubted, or decided tobe present in the baggage in step 202, the program goes to thehigh-selectivity detailed checking mode. If the decision in step 204 isalso made after the accumulation of 10 measurements each of which isselected to take 0.5 second for step 203, the total detecting timeincluding the screening mode beginning with step 201 becomes about sixseconds. The baggage inspection associated with security gate shouldusually be completed in a few seconds including the times of thecarrying in and setting of the baggage in the detecting device,detecting it and the carrying out of it. Therefore, the possibledetecting time to be actually taken is in the range from one to twoseconds. However, since the object to be detected is supposed not to beplaced in most pieces of baggage, the detecting can be completed inabout one second by the screening mode. Accordingly, use of thealgorithm shown in FIG. 1 makes it possible to suppress the averagedetecting time to about 1˜2 seconds per piece of baggage even if thedevice takes the time necessary for the processes up to thehigh-selectivity detailed checking mode. Thus, the baggage inspectioncan be made without remarkably interfering with the flow of baggage atthe security gate. In addition, since the decision based on tandem massspectrometry is finally performed by the high-selectivity detailedchecking mode, the selectivity can be increased, and misreport can bereduced.

Since the detailed checking by tandem mass spectrometry takes much timeas described above, an alarm lamp should be turned on or a signal forthe operator to easily detect should be produced in the stage where theprogram goes to step 203 after the decision of step 202, or when theprogram shifts from the screening mode to the detailed checking mode.

FIG. 2 is a diagram showing the construction of the detecting device forembodying the invention. Here, a quadrupole ion trap mass spectrometer(thereafter, referred to as the ion trap mass spectrometer) is used forthe mass spectrometry portion. An ion source 20 is coupled to a gasintroduction tube 21, and exhaust pipes 22 a, 22 b. The gas from the gassampling port is sucked in by a pump connected to the exhaust pipes 22a, 22 b, and introduced through the gas introduction tube 21 to the ionsource 20. The ingredients contained in the gas introduced into the ionsource are partially ionized. The ions generated by the ion source andpart of the gas introduced into the ion source are fed through first,second and third apertures 25, and taken in a vacuum space 27 that isevacuated by a vacuum pump 26. These apertures have a diameter of about0.3 mm. The, electrodes of these apertures are heated up to a range fromabout 100° C. to 300° C. by a heater (not shown). The remaining gas notfed into the first aperture 23 is exhausted to the outside of the devicefrom the exhaust pipes 22 a, 22 b through the pump.

There are differential evacuation spaces 28, 29 between the apertureelectrodes 23, 24, 25, and these spaces are evacuated by a roughing pump30. The roughing pump 30 is usually a rotary pump, scroll pump ormechanical booster pump. A turbo-molecular pump may be used for theevacuation of these regions. The aperture electrodes 23, 24, 25 to whichvoltages are applied thus improve the ion transmittance and splitcluster ions generated due to adiabatic expansion by the collision withthe residual molecules.

In the device shown in FIG. 2, a scroll pump of 900 liters per min isused for the roughing pump 30, and a turbo-molecular pump of 300liters/min for the vacuum pump 26 that evacuates the vacuum space 27.The roughing pump 30 is also used dually as a pump for evacuating theback pressure side of the turbo-molecular pump. The pressure between thesecond and third apertures 24, 25 is about 1 torr. In addition, only thefirst and third apertures 23, 25 with the second aperture electrode 24removed may be used to produce the differential evacuation space.However, since the gas inflow increases as compared with the previouscase, it is necessary to increase the evacuation speed of the vacuumpump used and the distance between the apertures. Also in this case, itis important to apply a voltage between the apertures.

The generated ions are converged by a converging lens 31 after passingthrough the third aperture 25. This converging lens 31 is usually anEinzel lens formed of three sheets of electrode. The ions further passthrough a slit electrode 32. The ions passed through the third aperture25 are converged on the opening of the slit electrode 32 by theconverging lens 31. The neutral particles that are passed but notconverged collide with this slit portion, and thus they are not easy toarrive at the mass spectrometry portion side. The ions passed throughthe slit electrode 32 are deflected and converged by a double cylindertype deflector 35 that is formed of inner and outer cylinder electrodes33, 34 having a large number of apertures. The double cylinder typedeflector 35 deflects and converges the ions by using the electric fieldof the outer cylinder electrode that is leaked from the apertures of theinner electrode. The details are already disclosed in JP-A-7-85834.

The ions passed through the double cylinder type deflector 35 areintroduced into the ion trap mass spectrometer that is formed of a ringelectrode 36 and end cap electrodes 37 a, 37 b. A gate electrode 38 isprovided to control the timing of the ions incident to the spectrometer.Flange electrodes 39 a, 39 b are provided to prevent the ions fromarriving at quartz rings 40 a, 40 b that hold the ring electrode 36 andend cap electrodes 37 a, 37 b, charging the quartz rings 40 a, 40 b.

The inside of the ion trap mass spectrometer is filled with helium froma helium gas supply tube (not shown) so that it is maintained at apressure of about 10⁻³ torr. A mass spectrometer controller (not shown)controls the ion trap mass spectrometer. The ions introduced into themass spectrometer collide with helium gas to lose their energy, and arecaught by an AC electric field. The caught ions are ejected out of theion trap mass spectrometer in accordance with the m/z value of ions byscanning with the high-frequency voltage applied to the ring electrode36 and end gap electrodes 37 a, 37 b, and the ejected ions are detectedby a detector 42 through an ion ejecting lens 41. The detected signal isamplified by an amplifier 43, and then processed by a data processor 44.

Since the ion trap mass spectrometer has an ion trapping characteristicwithin the inside (the space surrounded by the ring electrode 36 and endcap electrodes 37 a, 37 b), the ions can be detected by increasing theion introduction time even if the amount of ions is small due to lowconcentration of the substance being detected. Therefore, even if thesample concentration is low, the ion trap mass spectrometer can increasethe concentration of the ions at a large magnifying power, and thuspretreatment (condensation) of sample can be very simplified.

FIG. 3 is a magnified view of the ion source portion of the device shownin FIG. 2. The gas introduced through the sample gas introduction tube21 is once introduced into an ion drift region 45. This ion drift region45 is substantially kept at atmospheric pressure. Part of the sample gasintroduced into the ion drift region is fed to a corona discharge space46, and the remaining portion is exhausted out of the ion source throughthe exhaust pipe 22 b. The sample gas introduced into the coronadischarge space 46 is ionized when it is brought in a corona dischargeregion 48 generated at around the tip of a needle electrode 47 byapplying a high voltage to the needle electrode 47. At this time, in thecorona discharge region 48, the sample gas is introduced in thedirection substantially opposite to the flow of ions that drift from theneedle electrode 47 to the opposite electrode 49. The generated ions areintroduced into the ion drift region 45 through an opening 50 of theopposite electrode 49 by an electric field. At this time, a voltage isapplied between the opposite electrode 49 and the first apertureelectrode 23, thereby drifting the ions so that the ions can beefficiently led to the first aperture 23. The ions introduced into thefirst aperture 23 are fed into the vacuum space 27 through the secondand third apertures 24, 25.

The amount of gas flowing in the corona discharge space 46 is importantfor high-sensitive and stable detecting. Therefore, a flow controller 51should be provided in the exhaust pipe 22 a. In addition, a heater (notshown) should be used to heat the drift region 45, corona dischargespace 46 and gas introduction tube 21 from the standpoint of samplesuction prevention. Although the amount of gas flowing in the gasintroduction tube 21 and exhaust pipe 22 b can be determined by thethroughput of a suction pump 52 such as a diaphragm pump and theconductance of the pipe arrangement, such a controller as the flowcontroller 51 shown in FIG. 3 may be provided in the gas introductiontube 21 or exhaust pipe 22 b. By providing the suction pump 52downstream relative to the ion generating area (namely, the coronadischarge space 46) as viewed from the gas flow, it is possible toreduce the effect of the contamination (suction of sample) within theinside of the suction pump 52.

FIG. 4 is a diagram showing one example of the sample-gas collectingportion of the device according to the invention. The detecting deviceis roughly divided into a main body 53, a gas suction unit 54, a case55, and the data processor 44. The gas suction unit 54 is connectedthrough the gas introduction tube 21 to a probe 56. The operator handlesthe probe 56 to bring it near a baggage or the like so that the airaround the baggage can be sucked and detected.

The operation of the ion trap mass spectrometer in the embodiment of theinvention will be described just for reference. FIG. 5 and FIG. 6 aretiming charts of the voltages applied to the ring electrode and end capelectrodes. FIG. 5 shows the operation in step 201 of FIG. 1, and FIG. 6shows the operation in step 203.

In step 201, in the ion trapping time interval, 302, a high frequencyvoltage is applied to the ring electrodes to generate an electric fieldfor catching ions within the mass spectrometer. In addition, the voltageapplied to the gate electrode is adjusted to control the ions to beintroduced through the gate electrode into the mass spectrometer. Then,in the analyzing time interval, 303, the voltage applied to the gateelectrode is adjusted to prevent the ions from further flowing in. Theamplitudes of the high-frequency voltages applied to the ring electrodeand end cap electrodes are controlled so that ions of different m/zvalues can be sequentially ejected out of the spectrometer within whichthe ions exist caught by the electric field. The detector then detectsthe ejected ions to produce a mass spectrum. In the residual ionremoving time interval, 301, the ions remaining within the massspectrometer are removed by turning off the voltage applied to the ringelectrode.

The ion trapping time interval 302, analyzing time interval 303 andresidual ion removing time interval 301 are selected to be typically0.04 second, 0.05 second and 0.01 second, respectively. Thus, the massspectrum can be produced once in 0.1 second. If the sample airconcentration is so rare to need high sensitivity, the ion trapping timeinterval 302 may be extended.

The operation in step 202 will be described with reference to FIG. 6.The operations in the ion trapping time interval 302 and analyzing timeinterval 303 are the same as in step 201. In the selection timeinterval, 304, ions of predetermined m/z values are selected from thevarious ions trapped in the ion trapping time interval 302, and causedto remain, and the other ions are removed. In this selection timeinterval 304, the filtered-noise field can be used that is disclosed in,for example, RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL 7, 1086-1089(1993). In the dissociation time interval, 305, the predetermined m/zions selected in the selection time interval 304 are energized andforced to collide with helium gas within the mass spectrometer toproduce fragment ions. In order to energize the ions, it is necessarythat a high frequency voltage be applied between the end cap electrodesto accelerate the ions within the mass spectrometer. When theaccelerated ions collide with the helium gas, part of the kinetic energyof the ions is converted to the internal energy of ions. During the timewhen the ions repeatedly collide with the helium gas, the internalenergy is accumulated to break the weak chemical bonds of the ions, thuscausing dissociation.

Since the tandem mass spectrometry has ion loss in the time intervals ofselection and dissociation, it is necessary to trap sufficient amountsof ions in the ion trapping time interval 302 in order to acquiresatisfactory mass spectra of fragment ions. Therefore, the ion trappingtime interval 302, analyzing time interval 303, selection time interval304, dissociation time interval 305 and residual ion removing timeinterval 301 are selected to be typically 0.40 second, 0.05 second, 0.03second, 0.01 second and 0.01 second, respectively. Thus, the massspectrum is acquired once in 0.5 second.

In the field of analysis, the mass spectrometry is used for variousintended purposes. When the mass spectrometry is used for the detectingdevice, however, there are different points from the normal analysis.

While the normal analysis treats a very large number of ingredients, thedetecting device detects extremely limited substances. Selecting anddetecting a few main components can find a bomb, for example, which isproduced by mixing various different detonating explosives. In addition,the normal analysis determines the value of the concentration ofsubstance, while determination of only the presence or absence of theobject suffices for the detecting.

Thus, the method of operating the mass spectrometer particularlyeffective in the detecting device will be described with reference toFIG. 6. When the trapped ions are ejected out of the ion trap massspectrometer, the ejection efficiency varies depending on the massscanning speed. In other words, in the analyzing time interval 303 ofFIG. 5, when the rate of increase of the amplitude of the voltageapplied to the ring electrode is increased so that the analyzing timeinterval 303 can be completed in a short time, the amount of ionsejected out of the spectrometer and arriving at the detector increasesand thus the sensitivity is improved. However, when the rate of increaseof the voltage amplitude applied to the ring electrode is raised in theanalyzing time interval 303, the mass resolution is reduced, and it isnot known exactly when the ions are ejected. Thus, the measured m/zvalues deviate from the correct values. For this reason, when the iontrap mass spectrometer is operated, a step 207 for selecting particularions (corresponding to the selection time 307) is provided between astep 206 for ion trap (corresponding to the ion trapping time interval302) and a step 208 for ejecting ions (corresponding to the analyzingtime interval 303) as shown in FIG. 7. That is, limiting the m/z of ionsremaining within the mass spectrometer compensates for the reduction ofthe resolution due to the fast mass scanning. Specifically, whenamphetamine is detected, the m/z-136 positive ions are first checked.Thus, in step 206 the ions generated by the ion source are trappedwithin the mass spectrometer, and in step 207 the ions other than them/z-136 ions are removed and the m/z-136 ions are selectively caused toremain. Then, in step 208 for ejecting ions, fast mass scanning is madeso that the ions remaining within the mass spectrometer can beeffectively ejected out of the mass spectrometer. Thus, since them/z-136 ions definitely arrive at the detector, precise mass selectionis not necessary in the step 208, so that the analyzing time can bereduced and that high-sensitive detecting can be performed. This methodis effective not only for the screening time but also for the case offully examining by tandem mass spectrometry. A step 210 for selectingthe m/z of ions remaining within the mass spectrometer can be providedbetween a step 209 for dissociation and the step 208 for ejecting ionsas shown in FIG. 8.

Moreover, in this method, since the kind of object to be detected islimited, the database of objects is created on the data processor, andeffectively used for the detecting. More specifically, in the tandemmass spectrometry, the optimum values of the length of the dissociationtime and the amplitude of the high frequency voltage applied to the endcap electrodes to energize the ions at the time of dissociation aredependent upon the chemical substance to be detected. Therefore, a step211, as shown in FIG. 9, is provided to examine the optimum analysisconditions for the components of each detected object to create adatabase, and when the operator feels the suspicion of the presence of aparticular substance in the fast mode, to refer to the database at thetime of shifting to the detailed checking mode so that the optimumanalysis conditions for this substance can be read in. Thus, asatisfactory mass spectrum of fragment ions can be obtained, and precisedecision can be made. For example, the optimum analysis conditions foreach of various banned chemicals such as amphetamine and cocaine areexamined to create a database, and the analysis conditions foramphetamine when amphetamine is suspected or for cocaine when cocaine issuspected are read out from the database and analyzed.

While the substance to be detected is ionized and followed by thescreening based on the m/z of ions in the above description, thescreening can be performed without necessarily specifying the m/z ofions by mass isolation. FIGS. 10 and 11 are diagrams useful forexplaining the second embodiment of the invention in which the screeningis made according to the ion current value. FIG. 10 is a timing chart ofhigh frequency voltage applied to the ring electrode and end capelectrodes. Although the ion trapping time interval 302 and ionselection time interval 304 may be sequentially provided as in FIG. 6,an ion trapping and selection time interval 302, 304 for simultaneouslytrapping and selecting ions is provided as in FIG. 10. FIG. 11 shows thefrequency of the high frequency voltage applied to the end capelectrodes at the trapping/selection time interval 302, 304. The ionstrapped within the ion trap mass spectrometer have easy-to-resonatefrequencies according to the m/z. Thus, when a signal includingfrequency components other than those (f1, f2, f3) corresponding to them/z of the ions of the substance to be detected is applied to theelectrodes, only the ions having the target m/z are trapped at thetrapping/selection time interval 302, 304. Then, an ion ejection timeinterval 306 is provided, and the ions remaining within the massspectrometer are ejected in the ion ejection time interval, and detectedby the detector. Here, when any signal is obtained, an alarm is issuedor more detailed check mode is brought about. In this method, no time istaken for the mass spectrometry, and thus faster screening can be made.

The third embodiment of the invention that is effective for detectingexplosives will be described with reference to FIGS. 12 and 13. Thisembodiment is a screening method using ions derived from nitro groupsobtained by dissociation of explosive. Energizing easily decomposes thenitro compound, and the nitro group tends to easily dissociate.Therefore, as shown in FIG. 12, a trap/dissociation time interval 302,305 for simultaneously trapping and dissociating ions is provided. FIG.13 shows the frequency of the high frequency voltage applied to the endcap electrodes in the trap/dissociation time interval 302, 305. A weaksignal including the frequencies (f1, f2, f3) corresponding to the m/zof ions of a substance to be detected, but not including a frequency(f4) corresponding to the m/z of ions derived from the nitro group beingdetected is applied to the end cap electrodes. When the ions of anexplosive being detected are trapped, they resonate with the highfrequency voltage applied to the end cap electrodes, and as a resultenergized to collide with helium. At this time, the nitro group isdissociated, and thus the ions derived from the dissociated nitro group,for example, of NO₂ ⁻ are detected in the analysis time interval 303,thus the screening being made. The merit of this method is that even ifthe sample also includes nitro compounds of other substance than thatbeing detected, the dissociated nitro group is detected, and thus theobject can be found.

In the above method, higher-level mass spectrometry may be made in orderto further increase the selectivity as shown in FIG. 14. In other words,a step 212 (called MS/MS/MS or MS³) may be provided that selects ions ofparticular m/z from the fragment ions, dissociates them and makes massspectrometry for the dissociated ions. These processes of selection,dissociation and mass spectrometry can be repeated until a satisfactoryselectivity can be obtained (generally this is called MS^(n)).

In this invention, the time required for the detecting in the fastscreening mode is different from that in the detailed check mode. Thus,as shown in FIG. 15, the detecting device according to the inventionshould be used together with a baggage carrying mechanism 57. Thecarrying mechanism 57, a carrying mechanism controller 58 and thedetecting device body 53 are connected by signal lines 59 a, 59 b. Inthe screening mode, the carrying mechanism carries pieces of baggage ata constant speed, but when the main body 53 is switched to the detailedcheck mode in which the detecting is made in a few seconds, the speed ofthe carrying mechanism 57 is controlled to be slow.

Thus, according to the invention, the detecting can be made with highspeed and with high selectivity, and thus the presence or absence ofsubstances to be detected can be examined without disturbing the flow ofbaggage and passengers.

1. A method for analyzing, comprising the steps of: ionizing a sample; dissociating plurality of precursor ions which have different m/z values; and deciding if fragment ions of a predetermined m/z value are present.
 2. The method for analyzing according to claim 1, further comprising the step of selecting ions from the fragment ions of the predetermined m/z value, dissociating the selected fragment ions, and analyzing the dissociated ions. 